Positron Emission Tomography (PET) is a functional imaging technique with potential to quantify the rates of biological processes in vivo. Many recent reports describe use of PET for patients with breast cancer utilizing positron-tagged tracers which allow tumors to be visualized with high sensitivity and specificity. However, current clinical PET scanners yield quite poor images of the breast due to low sensitivity, count rate limitations and large amount of scatter, and are also expensive. In the proposed project, we plan to develop a high resolution, dedicated, small volume PET system for breast imaging using a promising new design of silicon avalanche photodiode (APD) arrays. The Phase I project was very successful, and we fabricated 4x4 element APD arrays. These arrays showed high gain, low noise, high quantum efficiency, and good energy resolution when coupled to LSO scintillators. Timing, crystal identification, and depth of interaction measurements were also carried out with very good results. Thus, the feasibility of using APD arrays in a compact, low cost, small volume PET system was adequately demonstrated. The Phase II project will build on the successful Phase I results and will involve development high performance PET modules for both breast and small animal imaging. PROPOSED COMMERCIAL APPLICATIONS: Medical applications such as dedicated small volume PET systems, whole body PET scanners, SPECT and gamma camera, and surgical probes. other non medical applications like astronomy, non-destructive testing, biological and chemical detection, nuclear and high energy physics, materials research.